1. Field of Invention
The present invention pertains to the field of automatic transmissions for motor vehicles and, more particularly, to a method and system for providing a voting strategy for determining, during vehicle start up, a mode state that the transmission was in when the vehicle was turned off.
2. Background of the Invention
A traditional automatic transmission includes a transmission control device employed to control the transmission of a motor vehicle. The transmission control device is used to select several ranges, such as: Park, wherein the transmission is locked to prevent the vehicle from moving; Neutral, wherein the transmission allows the vehicle to be moved freely, such as when being towed; Reverse, wherein the transmission allows the vehicle to move backwards; and one or more Drive ranges that enable forward motion of the vehicle. Usually, the transmission control device is in the form of a lever linked with a mechanical connection, such as a cable or a hydraulic line, to the transmission. Typically, the lever is also connected to an indicator. As the transmission control mechanism is moved from one range to another, the mechanical connection physically shifts the transmission to the selected range and the indicator moves to show the driver which range has been selected. Even if the vehicle is turned off, the driver is able to determine the current transmission range from the indicator and, in some cases, move the transmission control mechanism to Neutral if, for example, the vehicle is to be towed.
The traditional automatic transmission utilizes multiple friction elements for automatic gear ratio shifting. Broadly speaking, these friction elements may be described as torque establishing elements, although they are more commonly referred to as clutches or brakes. The friction elements function to establish power flow paths from an internal combustion engine to a set of vehicle traction wheels. During acceleration of the vehicle, the overall speed ratio, which is the ratio of a transmission input shaft speed to a transmission output shaft speed, is reduced during a ratio upshift as vehicle speed increases for a given engine throttle range. A downshift to achieve a higher speed ratio occurs as an engine throttle range increases for any given vehicle speed, or when the vehicle speed decreases as the engine throttle range is decreased. Various planetary gear configurations are found in modern automatic transmissions. However, the basic principle of shift kinematics remains similar. Shifting an automatic transmission having multiple planetary gearsets is accompanied by applying and/or releasing friction elements to change speed and torque relationships by altering the torque path through the planetary gearsets. Friction elements are usually actuated either hydraulically or mechanically based on the position of the transmission control device.
In a shift-by-wire transmission arrangement, the mechanical connection between the transmission control device and the transmission is eliminated. Instead, the transmission control device transmits an electrical signal along a wire to an electronic controller, which directs separate actuators to apply or release the various friction elements to obtain a desired gear ratio. The control device is no longer necessarily in the form of a lever because the control device is no longer moving a mechanical connection for controlling the transmission. Instead, the control device is typically an electro-mechanical interface (e.g., a series of buttons, a lever or a knob) that is used to instruct the transmission to switch between the transmission ranges. An electronic display, powered by a battery on the vehicle, is typically employed to indicate the current range for the transmission. Obviously, the electronic display must be on, and thus drawing power, in order for the driver to know which range has been selected.
Many vehicles with a shift-by-wire transmission incorporate a “Return to Park” feature to automatically shift the transmission into Park. See, for example, U.S. Pat. Nos. 3,937,105, 4,892,014 and 7,156,218, all of which are incorporated herein by reference. Such a feature is activated when certain triggering events occur, for example, when the system detects a seat belt being unbuckled while a driver door is opened and the vehicle is nearly stopped, or when the ignition is turned off. Automatically shifting the transmission into Park prevents unwanted motion of the vehicle.
While such a feature is helpful under most driving conditions, the feature is not helpful when the driver would like to exit the vehicle and have the vehicle remain in Neutral. For instance, many car-washing facilities are designed to pull a vehicle through a car wash machine with a conveyor. The driver simply drives the vehicle up to the conveyer, leaves the transmission in neutral and then exits the vehicle. The vehicle is then washed as the vehicle is either pushed or pulled through the car washing machine. However, if the vehicle automatically shifts to Park as soon as the driver exits the vehicle, both the vehicle and the conveyor may suffer damage.
Based on the above, there has been proposed a system for providing a Neutral Hold mode for a shift-by-wire transmission that is placed in various shift ranges, such as Park, Drive, and Neutral, and has an automatic Return to Park feature for shifting the transmission to Park when the vehicle is traveling at an extremely low speed or in a stopped condition as a driver prepares to exit the vehicle. The system includes a transmission control mechanism for determining which of the transmission ranges or modes is desired by the driver. A controller is used to receive information from various sensors, such as a door opening sensor, a seat belt sensor, an ignition switch sensor and the transmission control mechanism, to determine then to shift the transmission into the various ranges and modes. The Neutral Hold mode keeps the transmission from automatically shifting to Park for relatively short periods of time in order to allow the vehicle to be moved, such as when on a conveyor of a car washing machine.
Preferably, there are several methods of entering the Neutral Hold mode. The controller is configured to maintain the transmission in Neutral and not shift the transmission to Park upon detecting that a driver has exited the vehicle and a message is displayed when the system has entered the Neutral Hold mode. The controller is configured to cease displaying the message upon detecting that a battery is providing power below a threshold level and the controller is configured to maintain the Neutral Hold mode when the battery ceases to provide power. The system will leave the Neutral Hold mode when the transmission control mechanism is used to select a range other than Neutral.
In many vehicles, mainly because of regulatory requirements, the electronic display must be on when the transmission is in any gear setting other than Park. Therefore, if the driver puts the transmission in Neutral in order to tow the vehicle, the electronic display will remain on and eventually drain the battery as the vehicle is being towed for long distances, for example when the vehicle is being towed behind a recreational vehicle on a cross country trip. There has been proposed a system for providing a Neutral Tow mode for a shift-by-wire transmission that is placed in various shift ranges, including Park, Drive, and Neutral, and has an automatic Return to Park feature for shifting the transmission to Park when the vehicle is traveling at an extremely low speed or in a stopped condition as a driver prepares to exit the vehicle. The system includes a transmission control, mechanism for determining which of the transmission ranges or modes is desired by the driver. A controller is used to receive information from various sensors, such as a door opening sensor, a seat belt sensor, an ignition switch sensor and the transmission control mechanism to determine when to shift the transmission into the various ranges and modes.
A Neutral Tow mode keeps the transmission from automatically shifting to Park for relatively long periods of time and does not require that the system be powered in order to allow the vehicle to be towed for long distances without draining the battery. The Neutral Tow mode is entered by the controller following a two-stage process wherein first a Neutral Tow option is displayed and then, when the option is selected, the Neutral Tow mode is engaged. The electronic display displays a Neutral Tow option when the transmission control mechanism has been used to select Park, the ignition switch is on, and the engine is off or an information button has been pushed. The display is active only for a limited amount of time after the Neutral Tow mode is entered, a door is opened, or the ignition switch is turned on so that power is not drained from the vehicle. The controller is further configured to enter the Neutral Tow mode upon detecting when the Neutral Tow option has been selected, when the foot brake has been depressed, and when the transmission has been shifted to Neutral. The controller is configured to keep the transmission in Neutral and not automatically shift to Park upon detecting a triggering event and configured to cancel the Neutral Tow mode upon detecting a certain sequence of events. The triggering event is preferably constituted by detecting that an ignition switch has been turned off, the driver has exited the vehicle or a parking brake has been released. The sequence of events includes at least two of the group consisting of: turning on the ignition switch; depressing and holding a brake pedal; and selecting Park.
As can be seen by the above discussion, there is a need in the art for a system that allows a vehicle with a shift-by-wire transmission to be placed in a mode which maintains Neutral without a driver present such as when the vehicle is carried by a conveyor during washing. In addition, there exists a need for a way to keep the transmission in the Neutral gear setting and to further not cause the battery to be drained during towing. It is important that these modes be maintained through the vehicle being powered down, battery disconnect, control module resets, control modules being replaced in service, etc. Typically, the transmission could be in many different mode states. For example, the transmission could be in a Default or Normal mode, or a Neutral Hold, a Neutral Tow mode. When the control module is turned off, the module may not be able to determine what mode the transmission was in before the module was turned off. Therefore, upon restart, the module may put the transmission in the wrong mode, resulting in a false entry or exit into Neutral Hold or Neutral. Tow mode. False entry into Neutral Hold or Neutral Tow from a default mode will result in the vehicle not being commanded to Park when the driver exits the vehicle, thereby resulting in unwanted movement of the vehicle. On the other hand, the false exiting from a Neutral Hold or Neutral Tow to a default mode will result in Park being commanded while the vehicle is being towed, resulting in the vehicle being dragged when it should have stayed in Neutral.
Relying on a single control module to store information regarding which mode the transmission is in can result in failures that cause a mode change because, at least under certain circumstances, the control module could “forget” what mode state the transmission is supposed to be in. For instance, if the control module stores the information in volatile memory, then a battery disconnect could result in loss of memory of the mode state. The control module could store data in non-volatile random access memory to solve problems occurring when the battery is disconnected but the control modules are often set up to write data to non-volatile memory at shut down, so a module reset could also result in loss of memory of the mode state. Even if non-volatile random access memory is written to carefully avoid memory loss, there are module failures that result in non-volatile random access memory errors, all resulting in loss of memory of the mode state. With this in mind, there is seen to exist a need in the art to accurately determine which mode state the transmission is supposed to be in when the transmission controller is turned on after being turned off normally or after a power failure or failure of one of the transmission control modules.